docs: def _clean_datetime(self, obj):
          """Python objects want to be text."""
          if isinstance(obj, datetime):
              # if it's not naive, put it on zulu time first:
              if obj.tzinfo is not None:
                  obj = obj.astimezone(pytz.utc)
              return obj.isoformat()[:self.MAX_LENGTH]
          if isinstance(obj, date):
              return obj.isoformat()